The present invention relates to flexible polymeric film packaging structures with improved oxygen and moisture barrier characteristics. More specifically, the invention relates to a film structure which includes a polymeric substrate, an oxygen barrier containing cross-linked polyvinyl alcohol ("PVOH") and a moisture barrier including a metallized polymeric composite.
Certain polymeric films employed for the packaging of foods inherently permit the transmission of oxygen and moisture from the outside of the film to the inside of a food package wrapped with the film. Oxygen and/or moisture permit rapid deterioration of foods packaged in polymeric films. Exclusion of oxygen and moisture from packaged foods retards product spoilage. Therefore, film structures which provide a barrier to oxygen and moisture are highly desirable food packaging materials.
Attempts have been made in the past to provide polymeric films which have high oxygen and moisture barrier. Many of the oxygen and water vapor transmission resistant films disclosed in the prior art provide films comprising polymeric substrates coated with PVOH coatings. Polyvinyl alcohols are polymers containing (--CH.sub.2 --CHOH--) groups. They can be applied from water solution and are in many respects attractive materials for use as barrier coatings for thermoplastic polymer substrates. When applied to thermoplastic polymer surfaces which have been treated by any of a number of means to render them receptive to coatings, polyvinyl alcohols adhere tenaciously, are flexible, clear and transparent, provide an oxygen impermeable barrier, and are resistant to permeation by greases, oils and a wide variety of common organic solvents, such as esters, ethers, ketones, hydrocarbons and chlorinated hydrocarbons.
The advantage of water solubility in the application of PVOH as a barrier coating becomes its chief drawback, however, when the barrier coating has been formed on a thermoplastic polymeric substrate. The poor water resistance of PVOH restricts its use as barrier coating to those few applications where nearly anhydrous conditions prevail. Moisture may act to cause staining or wear due to friction, an unpleasant feel to the touch and a dull appearance. It is also known that the oxygen permeability of PVOH increases in proportion to its moisture content.
A number of methods have been known for increasing the water resistance of PVOH, but none provides entirely satisfactory results. In spite of various heretofore practiced cross-linking methods, the polyvinyl alcohol still tends to soften, swell and lose adhering strength when exposed to water or even moisture vapor. Hence, barrier coatings consisting only of polyvinyl alcohol cannot be used where a substantial degree of water-resistance or moisture-proofness is required.
U.S. Pat. No. 3,282,729 to Richardson, et al., discloses thermoplastic polymer substrates coated with polyvinyl alcohol cross-linked with a trimethylolphenol compound such as 2, 4, 6-tris (hydroxymethyl) phenol. The polyvinyl alcohol and the trimethylolphenol compound are applied to the thermoplastic substrate in a water solution. The coated substrate is then cured at elevated temperature to allow the cross-linking of the two compounds with each other. To cure the PVOH coating, Richardson et al. use high temperatures for extended periods of time, providing a process that is energy intensive and thus expensive. Moreover, there is no indication in the '729 reference to use a metallized polymeric composite to enhance the moisture barrier characteristics of the packaging film.
Accordingly, there is still a need in the art of packaging films to provide a film structure which has both excellent oxygen and moisture barrier characteristics.
It is, therefore, an object of the present invention to provide a new packaging film structure which employs a cross-linked PVOH layer exhibiting both excellent oxygen barrier and moisture barrier characteristics, even in the presence of water.